1. Field of the Invention
The present invention relates to a switching power supply system which converts a DC voltage, obtained by full-wave rectification of an AC voltage, to a desired DC voltage by carrying out turning-on and -off of the DC voltage obtained from the AC voltage by means of a switching device and outputs the converted DC voltage. The invention particularly relates to a switching power supply system in which a power factor improving operation is carried out in. current mode control.
2. Description of the Background
In a switching power supply system in which a power factor improving operation is carried out in current mode control, a DC voltage obtained by full-wave rectification of the voltage of an AC (Alternating Current) power supply is taken as an input voltage, and control is carried out so that the waveform of a current flowing in an inductor in the switching power supply system becomes a waveform proportional to (and in phase with) the voltage waveform of the input voltage, so that the power factor is improved.
An example of the configuration of such a switching power supply system in which a power factor improving operation is carried out is shown in FIG. 4. FIG. 4 is a diagram showing an example of the configuration of a step-up type switching power supply system in which a power factor improving operation is carried out in current mode control (a method of detecting the magnitude of a current flowing in a switching device to carry out control on the basis of the result of the detection). Current mode control is a control method applied in the case when monitoring of the peak value of a current flowing in a switching device is necessary, such as in overcurrent control. In FIG. 4, a diode bridge DB carries out full-wave rectification of the voltage of a AC power supply AC. A switching power supply, in which a DC voltage Vin obtained by the full-wave rectification of the AC voltage of the AC power supply AC with the diode bridge DB is taken as an input voltage, has an input capacitor Ci, an inductor L1, a transistor QSW as a switching device, a diode D1 as a commutating device, an output capacitor Co, a resistor Rs as a current detecting element, a resistor Rfilt and a capacitor Cfilt forming a filter circuit, an output terminal Vout (the output voltage at the terminal is also referred to as Vout) and a terminal GND (a terminal at the ground potential), voltage dividing resistors R20 and R21 for dividing the output voltage Vout, voltage dividing resistors R30 and R31 for dividing the full-wave rectified DC voltage Vin as the output of the diode bridge DB, a power supply controlling IC (Integrated Circuit) 10, and a resistor Rcomp and capacitors Ccomp1 and Ccomp2 forming a phase compensation circuit.
In the switching power supply system, the energy stored in the inductor L1 when the switching device QSW is turned-on is released through the diode D1 from the inductor L1 to the outside including the output capacitor Co when the switching device QSW is turned-off thereby to realize a step-up operation (the operation of a step-up type switching power supply system is well-known, so that a detailed explanation of the system will be omitted). The input capacitor Ci is for removing ripples associated with the switching of the switching device QSW. The current flowing in the switching device QSW flows in the resistor Rs as it is to develop a voltage (a current detecting signal), which is proportional to the current flowing in the switching device QSW, across the resistor Rs. The current detecting signal, with the noise associated with the switching of the switching device QSW removed by passing through the filter circuit formed with the resistor Rfilt and the capacitor Cfilt, is inputted to an input terminal IS of the power supply controlling IC 10. Moreover, the voltage, into which the output voltage Vout is divided by the voltage dividing resistors R20 and R21, is inputted to an input terminal FB of the power supply controlling IC 10. The voltage into which the full-wave rectified DC voltage Vin is divided by the voltage dividing resistors R30 and R31, is inputted to an input terminal MUL. An output terminal OUT of the power supply controlling IC 10 is connected to the gate of the switching device QSW for controlling the on and off of the switching device QSW by the output signal from the output terminal OUT.
The power supply controlling IC 10 has a flip-flop FF1, a set voltage circuit V10 (the set voltage as its output is also referred to as V10), an error amplifier OP3 amplifying the difference between the signal inputted to the input terminal FB and the set voltage V10, a multiplication circuit 1 to which the output of the error amplifier OP3 and the signal from the input terminal MUL are inputted, a driver 2, an overcurrent protecting circuit 3 outputting a reference signal for deciding an overcurrent so as to carry out overcurrent protection, a slope compensating circuit 4 generating a monotonously increasing signal (a slope compensating signal) to output a signal as a (weighted) sum of the slope compensating signal and the current detecting signal for carrying out later explained slope compensation , an oscillator 5 for setting the flip-flop FF1 at periodic intervals, and a PWM comparator CP1 comparing lower one of the voltage of the reference signal (voltage signal) outputted from the overcurrent protecting circuit 3 and the output voltage of the multiplication circuit 1 with the output voltage of the slope compensating circuit 4. The output terminal of the error amplifier OP3 is also connected to a terminal COMP of the power supply controlling IC 10, on the outside of which the phase compensation circuit, formed of the resistor Rcomp and the capacitors Ccomp1 and Ccomp2, of the error amplifier OP3 is connected to the terminal COMP. Since the switching power supply system is operated so that two signals inputted to the error amplifier OP3 become equal to each other, the output voltage Vout becomes a voltage with the value for which the value of the set voltage V10 is multiplied by the reciprocal of the voltage dividing ratio provided by the voltage dividing resistors R20 and R21. The flip-flop FF1 is set by the oscillator 5 at periodic intervals and reset at the time when the output voltage of the slope compensating circuit 4 becomes higher than the lower one of the voltage of the reference signal outputted from the overcurrent protecting circuit 3 and the voltage of the output of the multiplication circuit 1. The output Q of the flip-flop FF1 is inputted to the gate of the switching device QSW through the driver 2 and the output terminal OUT to control the turning-on and -off of the switching device QSW. Ordinary control is carried out by comparing the output of the multiplication circuit 1 and the output of the slope compensating circuit 4 to operate the whole circuit so that the output signal of the slope compensating circuit 4 based on the current detecting signal becomes identical with the output of the multiplication circuit 1 proportional to the voltage with a waveform of full-wave rectification of an AC voltage, by which a power factor is made improved. Moreover, when the output of the multiplication circuit 1 becomes excessive, the reference signal outputted from the overcurrent protecting circuit 3 prevents the current flowing in the switching device from becoming excessive. In current mode control, when a ON duty (=on-duration/switching period (=on-duration+off-duration) of the switching device QSW is 50% or more, subharmonic oscillation sometimes occurs to cause the operation to be unstable. As a measure for improving the operation becoming thus unstable, slope compensation is provided.
In the following, subharmonic oscillation and slope compensation will be explained. First, when the switching power supply system is in a stable state or in an equilibrium state, an increase in an on-duration and a decrease in an off-duration in the current in the inductor L1 are equal to each other. Moreover, the ON duty being 50% or more means that the increasing rate (m1) in the current in the inductor L1 in an on-duration (between certain times t1 and t2) is smaller than the absolute value (m2) of the decreasing rate (−m2) in the current in the inductor L1 in an off-duration (between the time t2 and a certain time t3), i.e. m1<m2. Here, consider the case in which the value of the current in the inductor L1 at the time t3 deviates from that at the time t1. The value of the current in the inductor L1 at the time t3 is equivalent to the value of the current in the inductor L1 at the time t1 in the next switching period. However, it is shown that when m1 and m2 are in a relation expressed as m1<m2, the deviation between the value of the current at the time t1 in a certain switching period and the value of the current at the time t1 in the next switching period increases (see JP-A-2004-40856, for example). This is known as a subharmonic oscillation. Conversely, with m1>m2, the deviation in the value of the current at the time t1 decreases in each switching period, so that the subharmonic oscillation can be inhibited. Therefore, for reversing a relation originally being m1<m2, a slope compensating signal (its inclination or its differentiation about time is taken as m3) produced by the slope compensating circuit 4 is added to a current detecting signal (with the inclination m1) to satisfy m1+m3>m2, by which the occurrence of a subharmonic oscillation is inhibited. This is known as slope compensation. The switching power supply system, which converts a DC voltage by means of a switching device to a desired DC voltage to output the converted DC voltage, is also provided with an over voltage protection circuit in addition to the overcurrent protection circuit for protecting the circuit in the system from an abnormal state.
Moreover, a proposal is also presented, in which, in a switching power supply system in which a power factor improving operation is carried out, even when the inputted voltage is high, reduction in power factor due to slope compensation is suppressed (see JP-UM-A-5-9187, for example) When the switching power supply system is formed of a step-up converter and when the slope compensating signal amplitude is fixed, the power factor sometimes is impaired more in a 200V input voltage system than in a 100V input voltage system. If the input voltage to the step-up converter is expressed by Vin and the output voltage from the step-up converter by Vout, the ON duty will be given by (Vout−Vin)/Vout. In other words, if the switching frequency is the same, the on-duration will be longer in the 100V input voltage system than in the 200V input voltage system. Since the increasing rate of the current flowing through the inductance L1 is proportional to the input voltage Vin, the increment of the current flowing through the inductance L1 in one on-duration is proportional to A=Vin·(Vout−Vin)/Vout. The current increment is large in the 100V input voltage system and small in the 200V input voltage system depending on the value of the output voltage Vout. If the output voltage Vout is 250V, the above-described A is 60 for the input voltage Vin of 100V and 40 for the input voltage Vin of 200V. If the slope compensating signal amplitude is fixed in the state, in which the increment of the current flowing through the inductance L1 is small under a high input voltage, the ratio of the slope compensating signal to the current detecting signal will be higher in the 200V input voltage system than in the 100V input voltage system. Therefore, the power factor is impaired more badly in the 200V input voltage system than in the 100V input voltage system. As a countermeasure against impairing the power factor, the power supply system disclosed in JP-UM-A-5-9187 employs different slope compensating signal amplitudes for the respective 100V and 200V input voltage systems. The slope compensating signal amplitude for the 100V input voltage system is set to be different from the slope compensating signal amplitude for the 200V input voltage system so that the ratio of the slope compensating signal to the current detecting signal in the output signal from the slope compensating circuit 4 may not be high in the 200V input voltage system.
In a related switching power supply system in which a power factor improving operation is carried out, when the system is operated in a current mode, a slope detecting signal was simply added to a current detecting signal in a power factor improving circuit. However, in the related switching power supply system described above, when the system was operated in a current mode, the slope compensating signal, monotonously increasing in the duration in which the switching device was turned on, was added to the current detecting signal in the power factor improving circuit. This has caused an overcurrent protection level to vary depending on an input voltage to be a problem of degrading accuracy of the overcurrent protection. This will be explained with reference to FIG. 5. FIGS. 5A and 5B are views illustrating the comparison carried out by the PWM comparator CP1 shown in FIG. 4. The comparison is carried out about the output of the slope compensating circuit 4 and the reference signal Vocp outputted from the overcurrent protecting circuit 3. In this case, the voltage of the output of the multiplication circuit 1 is to be higher than the voltage of the reference signal Vocp. As was explained in the foregoing, the PWM comparator CP1 compares the lower one of the voltage of the reference signal Vocp and the output voltage of the multiplication circuit 1 with the output voltage of the slope compensating circuit 4. Therefore, the output of the multiplication circuit 1 is negligible in the following explanation. FIG. 5A shows the case in which the input voltage Vin (equivalent to the output voltage of the diode bridge DB) to the switching power supply system is low and FIG. 5B shows the case in which the input voltage Vin is high (two times the voltage in the case shown in FIG. 5A). In the figures, a thin solid line represents a monotonously increasing current detecting signal and a thick solid line represents an output signal of the slope compensating circuit 4 as a sum of a monotonously increasing current detecting signal and a monotonously increasing slope compensating signal. The difference between the value represented by the thick solid line and the value represented by the thin solid line is equivalent to an amount of slope compensation. In the system, even though the level of the current detecting signal in (a) and that in (b) are equal to each other at the time when the switching device QSW is turned on, the level of the current detecting signal when the level of the output of the slope compensating circuit 4 reaches the level of the reference signal Vocp to be decided that an overcurrent is detected in FIG. 5A becomes different from that in FIG. 5B. Therefore, to the reference signal Vocp of the same level, currents with their levels different from each other are decided as being overcurrents.
Specifically, the current flowing in the switching device QSW is the current IL1 flowing in the inductor L1. Thus, when the on-resistance and the resistance Rs (Rs is low) of the switching device QSW is neglected, the increasing rate of the current IL1 flowing in the inductor L1 is proportional to the input voltage Vin (L1(dIL1/dt)=Vin). Therefore, as the input voltage Vin becomes higher, the output of the slope compensating circuit 4 reaches the level of the reference signal Vocp in a shorter time. Since the inclination of the slope compensating signal is independent of the input voltage Vin, as the time that elapses until the level of the output of the slope compensating circuit 4 reaches the level of the reference signal Vocp becomes shorter, the amount of the slope compensation, equivalent to the length between two arrows shown in FIG. 5, becomes smaller. The value of the net current detecting signal when it is decided that an overcurrent is detected is given as a difference obtained by subtracting an amount of slope compensation from the level of the reference signal Vocp. Therefore, the net current detecting signal becomes lower as the input voltage becomes lower. Thus, to the same level of the reference signal, currents with different values are decided to be overcurrents. In the power factor improving circuit, the use of a voltage, having a waveform obtained by full-wave rectification of the commercial AC, as the input causes the input voltage to be not constant. Thus, when the input voltage is low, an overcurrent protection level (the level of a net current detecting signal decided as being an overcurrent) becomes low. While, when the input voltage is high, the overcurrent protecting level becomes high. Moreover, an explanation from another view point will be given as follows. As was explained in the foregoing, the value of the net current detecting signal when it is decided that an overcurrent is detected is given as a difference obtained by subtracting an amount of slope compensation from the level of the reference signal Vocp. Therefore, as the time that elapses until the level of the output of the slope compensating circuit 4 reaches the level of the reference signal Vocp (the time during which the switching device QSW is turned on) becomes longer, the amount of the slope compensation becomes larger to cause the value of the net current detecting signal to become smaller. When the input voltage is low, the ON duty is large (The ON duty is expressed as (1−Vin/Vout) in a step-up type switching power supply and (Vin/Vout) in a step-down type switching power supply, for example. The expressions relate to the current IL1 flowing in the inductor L1 and are obtained from the fact that the amount of increase in the IL1 in an on-duration is equal to the amount of decrease in an off-duration. However, the detailed explanation about them will be omitted.) to make an on-period long (the switching period is constant independently of the input voltage Vin). When the input voltage is high, the ON duty is small to make the on-period short. Therefore, with the overcurrent protection level (the level of the reference signal Vocp) being constant, the lower the input voltage is, the higher the probability becomes that the time becomes longer that elapses until the level of the output of the slope compensating circuit 4 reaches the level of the reference signal Vocp, that is, the higher the probability becomes that the time becomes longer during which the switching device is turned on. Hence, the level of the net current detecting signal relating to overcurrent protection is liable to become small.
The invention was made in view of such points with an object of providing a switching power supply system that is capable of performing highly accurate overcurrent protection with the overcurrent protection level being independent of (unaffected by) an input voltage when the system is operated in a current mode.